1. Field of the Invention
The present invention relates to an electrophotographic toner, a developer, and an image forming apparatus.
2. Description of the Related Art
Conventionally, in an image forming apparatus of an electrophotographic type and the like, a latent image that is electrically or magnetically formed is visualized with the help of electrophotographic toner (which may be simply referred to as “toner,” hereinafter). For example, according to an electrophotographic method, an electrostatic charge image (latent image) is formed on a photoconductor, and the latent image is then developed with the toner. In this manner, a toner image is formed. The toner image is usually transferred onto a transfer material such as paper, and is then fixed on the transfer material. In a fixation process of fixing the toner image on the transfer paper, heat fixation methods, such as a heating roller fixation method and a heating belt fixation method, have been widely used because the heat fixation methods are good in energy efficiency.
In recent years, the market's needs for high-speed, energy-saving image forming apparatus have been further growing. What is desired is a toner that is excellent in low-temperature fixation performance and able to provide high-quality images. To achieve the low-temperature fixation performance of the toner, a softening temperature of binder resin of the toner needs to be made lower. However, if the softening temperature of the binder resin is low, a so-called offset (also referred to as “hot offset,” hereinafter) can easily occur as part of the toner image adheres to a surface of a fixation member during the fixation process, and is transferred onto copy paper as a result. Moreover, the heat-resistant storage stability of the toner decreases, and so-called blocking can occur as particles of the toner are fused together particularly in a high-temperature environment. Other problems also arise: In a developing unit, the toner is fused to the inside of the developing unit or a carrier, resulting in contamination; and the filming of the toner can easily occur on the surface of the photoconductor.
As for a technique for solving the above problems, the use of crystalline resin as the binder resin of the toner is known. That is, the crystalline resin softens rapidly at a melting point of the resin, and is able to bring the softening temperature of the toner down to around the melting point while ensuring the heat-resistant storage stability below the melting point. Therefore, if the crystalline resin is used in the toner, both the low-temperature fixation performance and the heat-resistant storage stability can be achieved.
For example, as a toner that uses the crystalline resin, a toner that uses, as binder resin, a crystalline resin whose crystalline polyester is extended with diisocyanate has been disclosed (see Japanese Patent Application Publication (JP-B) Nos. 04-024702 and 04-024703). Although the toner is excellent in low-temperature fixation performance, the problem is that the hot-offset resistance thereof is insufficient, and the quality thereof falls short of a level required in recent years.
Then, a toner using a crystalline resin that contains a sulfonic acid group and has a crosslinked structure by unsaturated bond has been proposed (see Japanese Patent (JP-B) No. 3910338). With the toner, there is an improvement in hot-offset resistance compared with conventional ones. Moreover, the technology of the following resin particles is disclosed (see Japanese Patent Application Laid-Open No. 2010-077419): the resin particles are excellent in low-temperature fixation performance and heat-resistant storage stability, with the ratio of softening temperature and heat-of-fusion peak temperature, and the viscoelastic properties defined.
Although the toner that uses the above crystalline resin as main component of the binder resin is excellent in impact resistance because of the properties of the crystalline resin, the toner is weak in indentation hardness such as Vickers hardness. Therefore, stirring stress in the development unit is likely to cause problems, such as contamination of the carrier or the inside of the unit, the filming on the photoconductor, and deterioration of electrification characteristics and liquidity caused by external additive buried. Moreover, it takes time for the toner that has melted on a fixation medium at the time of heat fixing to be crystallized again. Therefore, the hardness of the surface of the image cannot quickly recover. Thus, the problem is that, because of paper discharge rollers or the like during a paper discharge process after fixation, changes in gloss and scratches could occur on the surface of the image due to rollers' traces. Another problem is that, since the hardness is insufficient even after the hardness of the surface of the image has been recovered by the recrystallization of the toner, the image is vulnerable to scratching and rubbing.
Meanwhile, a technique for improving the stress resistance of the toner is disclosed (see JP-B No. 3360527): According to the technique, the durometer hardness of the crystalline resin is defined, and inorganic fine particles are contained in the toner.
However, the technique cannot be used to repair scratches that are caused by rollers' traces immediately after the fixation; the hardness of the image after the recrystallization is insufficient. Another problem is that the low-temperature fixation performance is significantly inhibited by inorganic fine particles, and the advantage of the fixation to the crystalline resin cannot be fully utilized.
A large number of techniques for using both crystalline resin and amorphous resin, instead of using crystalline resin as a main component of the binder resin, are disclosed (for example, see JP-B Nos. 3949526 and 4513627). The above toners can compensate for the disadvantage of the hardness of the crystalline resin with the amorphous resin. However, the problem is that the crystalline resin that is effective in terms of low-temperature fixation performance cannot be fully utilized.